The present invention relates generally to prosthetic joints and, more particularly, to a modular tibial augment for prosthetic knee joints. The tibial augment may be used to compensate for a broad range of types and sizes of bone deficiencies.
The knee joint basically consists of the bone interface of the distal end of the femur and the proximal end of the tibia. Appearing to cover or at least partially protect this interface is the patella which is a sesamoid bone within the tendon of the long muscle (quadriceps) on the front of the thigh. This tendon inserts into the tibial tuberosity and the posterior surface of the patella is smooth and glides over the femur.
The femur is configured with two knob like processes (the medial condyle and the lateral condyle) which are substantially smooth and articulate with the medial plateau and the lateral plateau of the tibia, respectively. The plateaus of the tibia are substantially smooth and slightly cupped thereby providing a slight receptacle for receipt of the femoral condyles.
When the knee joint is injured whether as a result of an accident or illness, a prosthetic replacement of the damaged joint may be necessary to relieve pain and to restore normal use to the joint. Typically the entire knee joint is replaced by means of a surgical procedure which involves removal of the ends of the corresponding damaged bones and replacement of these ends with prosthetic implants. This replacement of a native joint with a prosthetic joint is referred to as a primary total-knee arthroplasty.
On occasion, the primary knee prostheses fails. Failure can result from many causes, including wear, aseptic loosening, osteolysis, ligamentous instability, arthrofibrosis and patellofemoral complications. When the failure is debilitating, revision knee surgery may be necessary. In a revision, the primary knee prosthesis is removed and replaced with components of a revision prosthetic knee system.
In revision knee surgery or arthroplasty, bone loss on the proximal tibia can make it difficult to properly position and support the tibial component of the revision system on the proximal tibial surface. The prior art has addressed this problem by providing tibial wedges. Generally, the tibial wedges are placed between part of the distal surface of the tibial tray component and part of the proximal tibia to support part of the tibial tray on the tibia by augmenting part of the tibia. An example of such a tibial wedge is disclosed in U.S. Pat. No. 5,019,103.
Due in part to the fact that the size, shape and anatomy of virtually every patient is different, and the variability in the location and amount of bone loss on the proximal tibia, an extensive number of a variety of tibial wedges have been made available to the orthopedic surgeon. For example, a typical surgical kit will include tibial wedges of different thicknesses and different configurations for use on either the medial or lateral sides of the tibial.
Examples of components of two revision knee systems are illustrated in FIGS. 1–9. The components illustrated in FIGS. 1–5 are part of one prior art revision knee system, and the components illustrated in FIGS. 6–9 are part of a second prior art revision knee system. Both include tibial components 10 comprising tibial trays 12 and tibial stems 14. The tibial stems 14 extend out from the distal side 16 of the tibial tray 12. Each tibial tray 12 is symmetrical about a central plane 18 that divides the tray into a medial side 20 and a lateral side 22. The medial and lateral sides 20, 22 are mirror images of each other.
In the prior art components illustrated in FIGS. 1–5, the tibial tray 12 has two threaded through-bores, 24, 26, one on the medial side 20 and one on the lateral side 22. The threaded through-bores 24, 26 extend from the distal surface 16 to the proximal surface 28 of the tibial tray 12. These threaded through-bores 24, 26 are provided for the selective mounting of a tibial wedge or augment element to the distal side 16 of the tibial tray.
Tibial augment or wedge components for the prior system of FIGS. 1–5 are illustrated in FIGS. 4–5. The two tibial augments or wedges 30, 32 are mirror images of each other. Each has a proximal surface or side 34 that is juxtaposed with the distal side 16 of the tibial tray when the augment or wedge is mounted on the tray and a distal side or surface 36 that is intended to be juxtaposed with a resected tibial surface when implanted. Each illustrated augment or wedge 30, 32 has an outer edge 37, 38 shaped to follow a portion of the outer edge 40 of the tibial tray 12. Each augment or wedge 30, 32 also has a smooth through-bore 42, 44 extending from the distal surface 36 to the proximal surface 34 of the augment or wedge 30, 32. A typical through-bore 42, 44 for each augment or wedge is illustrated in cross-section in FIG. 5. As there shown, each smooth through-bore 42, 44 has a large diameter countersink 46 extending from the distal surface 36 toward the proximal surface 34 and a small diameter neck 48 at the proximal end of the countersink 46. In the system of FIGS. 1–5, the small diameter neck 48 is defined by a smooth surface. On the proximal surface 34, each through-bore 42, 44 includes an enlarged undercut 50. The augments or wedges 30, 32 can be mounted to the distal side 16 of the tibial tray 12 with screws (not shown); the heads of the screws are received in the countersinks 46, and the threaded screw shaft extends through the reduced diameter neck 48 to engage the threads of the threaded through-bores 24, 26 of the tibial tray 12. With the through-bores shaped as illustrated in FIG. 5, the augments or wedges 30 of FIG. 3 can only be mounted on one side of the central plane 18 of the tibial tray 12, on the side shown at 22 in FIG. 2. Similarly, the augment or wedge 32 shown in FIG. 4 can only be mounted on the other side of the central plane 18 of the tibial tray 12, on the side shown at 20 in FIG. 2.
Since each augment or wedge 30, 32 can only be mounted on one side of the central plane 18 of the tibial tray 12, it is necessary to provide two complete sets of augments or wedges in a typical surgical kit, one set for use on the medial side and one set for use on the lateral side. Each of these sets would typically include at least two different thicknesses of tibial augments or wedges. The typical surgical kit would also typically include several different sizes of tibial augments or wedges 30, 32, corresponding to the sizes of tibial trays provided. Thus, a substantial number of augments or wedges 30, 32 are needed in the revision system of FIGS. 1–5.
A substantial number of augments or wedges 30, 32 are also needed in the revision system of FIGS. 6–9. In the system of FIGS. 6–9, like reference numbers have been used for similar parts. In this prior art system, the tibial stem 14 and distal side 16 of the tibial tray 12 are connected by keels 52. Each augment or wedge 30, 32 includes two threaded through-bores 54, 56 and a cut-out 58 to complement and receive one of the keels 52. Each of the two sides 20, 22 of the tibial tray 12 has two threaded bores 58, 60 to be aligned with the threaded through-bores 54, 56 of one of the augments 30, 32. As shown in FIG. 9, each threaded through-bore 54, 56 of each augment has a proximal and distal countersink 62, 64 and an elongate threaded neck 66. To mount one of the augments or wedges 30, 32 on the tibial tray 12, a screw is threaded through each threaded through-bore 54, 56 of the augment and threaded into the threaded bore 58, 60 of the distal side 16 of the tibial tray 12.
Since the through-bores 54, 56 of the augments 30, 32 are threaded, each augment 30 or 32 can only be mounted on one side of the central plane 18 dividing the tray 12 into medial and lateral portions 20, 22. Since each augment or wedge 30, 32 can only be mounted on one side of the central plane 18 of the tibial tray 12, it is also necessary to provide two complete sets of augments or wedges in a typical surgical kit for the system of FIGS. 6–9, one set for use on the medial side and one set for use on the lateral side. Each of these sets would also typically include at least two different thicknesses of tibial augments or wedges. The typical surgical kit would also typically include several different sizes of tibial augments or wedges 30, 32, corresponding to the sizes of tibial trays provided. Thus, a substantial number of augments or wedges 30, 32 are needed in the revision system of FIGS. 6–9.
The need for a large number of augments or wedges in the surgical systems of the prior art adds substantially to the cost of these systems.